installer/build.ps1: update to use TLS 1.2 when downloading from github due to https://blog.github.com/2018-02-23-weak-cryptographic-standards-removed/, fixes usage on Win10
installer/build.ps1: update to use TLS 1.2 when downloading from github due to https://blog.github.com/2018-02-23-weak-cryptographic-standards-removed/, fixes usage on Win10

martinlindhe/wmi_exporter,martinlindhe/wmi_exporter